Is there a difference between an enum and a sealed interface that only contains objects? I wonder if...

# getting-started

[rednifre]

Is there a difference between an enum and a sealed interface that only contains objects? I wonder if sealed classes/interfaces make enums obsolete.

[Johann Pardanaud]

there are some differences, for example you can iterate the values of an enum but not the fields of a sealed class

also an enum value is a class, so it can contain some fields to enrich it

and finally, an enum allows you to work easily with when and enforces you to handle all the cases

[ephemient]

you can iterate Parent::class.sealedSubclasses.map { it.objectInstance } - there's an open issue to make it available without runtime reflection

[ephemient]

when works fine on objects (or any other subtype) of sealed classes and interfaces

[rednifre]

I’m not clear on “also an enum value is a class, so it can contain some fields to enrich it”, objects can also contain fields, right?

[ephemient]

the bytecode generated is a bit different (makes usage from Java significantly different) and only enums can be used in annotations

[ephemient]

otherwise I'd just go with whatever fits your usage better

[Joffrey]

Also, conceptually, a sealed class is an "enum" at the type level, while an enum is enumerated at the value level. Having only objects also makes this enumeration at the value level, but still there is this

[ephemient]

there was a conversation just a little earlier: https://kotlinlang.slack.com/archives/C0B8MA7FA/p1668286904827229?thread_ts=1668286904.827229&cid=C0B8MA7FA

[ephemient]

the fact that all the enum values are the same type means they can't have different members or extensions

[rednifre]

Oh, that’s a good point! Also the bit of only enums being allowed in annotations.

[ephemient]

(enum values can override the same members differently though)

[ephemient]

there's a little hopefully irrelevant implementation note:

enum class Example {
    Foo {
        override fun toString(): String = "foo"
    },
    Bar
}
Example::class.java != Example.Foo::class.java
Example::class.java == Example.Foo.declaringClass
Example::class.java == Example.Bar::class.java

[ephemient]

relevant: https://youtrack.jetbrains.com/issue/KT-49653/Deprecate-and-remove-Enum.declaringClass-synthetic-property

[ephemient]

also relevant to earlier comment: https://youtrack.jetbrains.com/issue/KT-25871/Provide-ability-to-enumerate-all-direct-subclasses-of-a-sealed-class-at-compile-time-without-reflection

[ephemient]

there's a sorta first party workaround, https://github.com/JetBrains-Research/reflekt/issues/71

[Stephan Schroeder]

TLDR: sealed classes are enums with superpowers

[Johann Pardanaud]

honestly, same here, sorry for the misleading comment earlier…

[rednifre]

Neat, thanks!